1. Field of the Invention
The present invention relates to an aluminum alloy sheet for superplastic forming. In particular, the present invention relates to an aluminum alloy sheet for superplastic forming of which the surface quality does not deteriorate during forming to provide a formed product with an improved quality and which exhibits improved scratch resistance when sliding occurs between a die and a material during forming.
2. Description of Related Art
In superplastic forming of an aluminum alloy sheet, an aluminum alloy sheet is generally formed at a high temperature by causing the aluminum alloy sheet to undergo a large amount of plastic deformation utilizing gas pressure. The superplastic forming method is suitable for small-quantity production of various products, since the aluminum alloy sheet can be formed into a complicated shape by processing the sheet once, and the die and the manufacturing equipment are inexpensive in comparison with those used for cold press forming.
In recent years, the cycle time of superplastic forming has been reduced through optimization of the forming conditions and equipment. Therefore, superplastic forming has also been applied to form mass-produced automotive outer panels. Since superplastic forming is a high-temperature treatment, a lubricant such as boron nitride (BN) or graphite is generally applied to the forming target aluminum alloy sheet to prevent the aluminum alloy sheet from adhering to the die or bead portion.
On the other hand, when forming mass-produced parts or formed products for which a high surface quality is required, scratches inevitably occur due to sliding between the die and the sheet, even if the lubricant is applied. The surface quality deteriorates due to deposition of the lubricant on the die. Moreover, it is difficult to automatize application of the lubricant. As a result, a decrease in efficiency and an increase in cost cannot be avoided.
In order to improve cold press formability, a method has been proposed in which an anodic oxide film is formed on the surface of the aluminum material and an organic resin film such as a polyurethane resin film in which a lubricant such as polyolefin wax fine powder particles are dispersed is formed on the anodic oxide film (see JP-A-8-187818). On the other hand, when applying this method to superplastic forming of an aluminum alloy sheet, a sufficient effect cannot be necessarily obtained.